1. Field of the Invention
This invention relates to an ink composition for waterless plates, which exhibits satisfactory resistance to scumming and excellent printability for use in waterless plate for lithography.
2. Prior Art
In lithography which is mainly used in the current printing field, printing is carried out by a process using a plate consisting of hydrophilic non-image area and oleophilic printing area and making use of the tendency that water and an oleophilic ink are repulsive to each other, wherein water is attached to the hydrophilic non-image area while an oleophilic ink is attached to the oleophilic printing area and then the oleophilic ink thus attached to the printing area is transferred to a to-be-printed substrate thereby obtaining a desired print.
In such conventional lithography, however, it is difficult to control the dampening solution which serves to prevent the ink from adhering to the non-image area. It is a serious technical obstacle to use such dampening solution, from the viewpoint of printability and printing effects. For that reason, there have been proposed many attempts to carry out a variety of waterless lithographic methods using waterless plates, which do not require such dampening solution. As the newest and practicable lithographic method using a waterless plate, there has been proposed a printing method using a plate in which the non-image area is covered with non-tacky thin films of silicone rubber or the like. It is reported that the printing method can perform printing with a common oleophilic ink without the supply of a dampening solution.
However, the reported method has now been found to be inappropriate, because it has a disadvantage such that scumming may occur when a conventional oleophilic ink is used for printing since the printability of the conventional ink is not suitable.
The phenomenon that scumming occurs in waterless lithography is due to the decreased cohesion potential of the ink of itself because the friction of the moving parts and rolls of the printing press used during printing raises the temperature of the plate, whereby the ink adheres to the non-image area which should originally be repulsive to ink.
Such phenomenon is a specific problem to occur only to waterless lithography. It is not at all required to take the phenomenon into consideration in case of the conventional lithography employing dampening solution. Since the conventional lithography employing dampening solution has a totally different repulsive mechanism of ink and furthermore the vaporization of dampening solution substantially suppresses the temperature rise of the print, it is not necessary to worry about such scumming phenomenon when using the conventional lithography. It is said that a lower cohesion of an ink may rather be preferable for the lithography.
To eliminate this drawback, there has been proposed a method comprising adding a silicone compound to an ink (Japanese Patent Gazette No. 50-11287, Japanese Patent Application Laid-open Gazette No. 55-31885), a method for modifying a resin with a silicone compound (Japanese patent Gazette Nos. 51-10124, 52-10041 and 52-10042), and the like. These methods, however, have not yet satisfactorily solved problems concerning scumming and have left other problems such as poor leveling of links, insufficient gloss, and the like.
There has also been proposed an improved method employing a varnish which is made into gell by incorporation with, for example, aluminum compound such as aluminum stearate, aluminum octoate, etc. When attempts are made to achieve high anti-scumming property by the above improved method, the ink used will greatly increase in viscosity, causing the problems such that the transferability of the ink, such as transfer of the ink on the printing press or to the plate or blanket, may be drastically depleted. That is, even the improved method is far from the stage satisfying the needs.